3-1 lecture 3: actinium chemistry lecture notes from radiochemistry of the rare earths, scandium,...
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3-1
Lecture 3: Actinium Chemistry
• Lecture notes from Radiochemistry of the Rare Earths, Scandium, Yttrium, and Actinium
• Chemistry of actinides§ Metals§ Soluble and insoluble salts§ Complex ions and chelate compounds§ Organic compounds§ Separations
3-3
Actinium Isotopes and fundamental data• Ac is trivalent
§ Very similar chemistry to lanthanides and Y§ Trivalent chemistry§ electropositive
• 227Ac§ From 235U decay§ Beta emitter
• 225Ac has been proposed for nuclear medicine§ Ac from 229Th parent
à Alpha emitting therapeutic agent§ Use of 225Ac to produce 213Bi
• Chemistry of Ac basis of use as radiopharmaceutical § Use of Ac in delivery to tumor based on fundamental chemical interactions§ Separation of Bi daughter from Ac parent
• Atomic radius§ 195 pm
• Ionic radius§ 126 pm§ 6-coordinate
3-4
Redox
• Trivalent is predominant state• Divalent state postulated based on similarities
to Ac behavior with divalent Eu and Sm§ Reinfored with polarogram data
à Two waves in HClO4, pH 1.9-3.1* Ac2++2e-+HgAc(Hg)* Ac3++3e-+HgAc(Hg)
§ Other experiments failed to find divalent state
3-5
Ac Metal• Preparation of metals difficult
§ Tendency to form oxides and hydroxides in water
§ Formation in electrolytic reductionà Molten salt systems
§ Reduction of fluoride salts by metallic Ca
§ Need to melt both CaF2 and resulting metal
• Metal oxidizes in air
§ 4Ac + 3O2 → 2Ac2O3
§ Useful starting material for synthesis
à Ac2O3 + 3H2S Ac2S3 + 3H2O
• Density§ 10.07 g/mL
• Melting point§ 1050 °C
• Boiling Point§ 3300 °C
• Crystal structure§ fcc
3-6
Ac preparation and purification
• Separation from U ores§ Ores also contain a fair amount of lanthanides,
require separation• Nuclear reactions and generators
§ Irradiation of 226Ra§ 226Ra(n,g)227Ra, beta decay to 227Ac
à 227Ac sg=762 barns§ 225Ac from 229Th generator
à Start with 233U from neutron activation of 232Th
3-7
Ac purification: solvent extraction• Extration with TTA• pH control of extraction
§ Ac extracts well above pH 6à Hydrolysis in this
range§ Synergist extraction
with 0.1 TTA in 0.1 M TBPà Above pH 4
Keto Enol Hydrate
aa
S
O O
CF3S
O OH
CF3S
OOH
CF3
HO
3-8
Ion exchange • Cation exchange
§ Separation of 227Ac from 227Th and 223Ra
§ Strong cation exchangeà DOWEX 50
• Organic stationary phases§ Trioctylamine§ Bis(2-ethyl-hexyl)phosphoric
acid (HDEHP)§ TBP§ TTA
• Inorganic
§ MnO2
3-9
Preparation of gram quantities of Ac
• Irradiation of multigram quantities of 226Ra§ Forms both 227Ac and 228Th§ 222Rn daughter from 226Ra
• Irradiation of RaCO3
§ Dissolved in dilute HNO3
à Ra(NO3)2 precipitated* Recycle for further Ac production
à Th and Ac remain in solution§ 5 M HNO3, anion exchange
à Th strongly absorbed, Ac only slightly§ Oxalate precipitation of Ac
à Calcination after precipitationà Used to form Ac2O3
3-10
Ac salts
• Salts are soluble in most acids§ Some salt are insoluble and used in separations based on
precipitation• Most data from one study
§ Each compound prepared from less than 10 µg§ Purified by TTA
• Hydroxides§ pK1h=9.4§ Described by electrostatic model of hydrolysis § Linearity of log K1h versus 1/ionic radius
à For trivalent metal ions, actually related to charge density
à Hydrolyzes less than trivalent lanthanides or actinides* More basic than lanthanides
Ac hydrolysis paper• http://www.ingentaconnect.com/content/klu/jrnc/2004/00000261/00000001/05379859;jsessionid=4dkgcbb4sv85c.alice
3-11
Ac salts• Fluorides
§ AcF3
§ Density 7.880 g/mL§ Formation of AcF3
à Ac(OH)3 + 3HF AcF3 + 3H2O • Oxalate
§ Oxalates used in precipitation of metal ions§ Oxalate salt of actinium used to form oxide§ Ac precipitated as an oxalate by the addition of an
oxalic acid § Oxalates are destroyed by boiling concentrated
HNO3 or HClO4 § In 0.1 M HNO3-0.5 M oxalic acid
à Ac oxalate solubility 24 mg/L
3-12
Ac salts• Chloride
§ Melting point: 1051°C (sublimes) § Density: 4.810 g/mL§ Formation reaction
à 4Ac(OH)3 + 3CCl4 4AcCl3 + 3CO2 + 6H2O • Bromide
§ Melting point: 1051°C § Boiling point: 3198°C § Density: 5.850 g/mL§ Formation reaction
à Ac2O3 + 2AlBr3 2AcBr3 + Al2O3 § For Iodide
à Ac2O3 + 2Al + 3I2 2AcI3 + Al2O3
3-13
Solubility and complexation• Generally co-precipitation with
insoluble salts from cation§ Fluorides, hydroxides of metal
ion§ Ac precipitated by Pb sulfate
• Solubility of oxalates evaluated§ Effects of radiolysis
à La solubility half of Ac solubility
à Ksp around 5E-27* Large decrease in pH
due to radiolysis from 227Th
* Purified Ac did not show large pH decrease
• Resembles lanthanum in complexation§ Generally lower for Ac
• Determined in tracer experiments§ Solvent extraction
• Compared to ionic radius to some evaluated constants§ HDEHP
3-14
Radiocolloid
• Separation of 227Ac from 227Th and 223Ra§ Formation of colliods
à Ac goes into solution, Th remains in solid phase§ Sorption of Ac onto filter increases with pH and time
à Above pH 5 filter separation of Ac by centrifuge
• Analytical chemistry of Ac§ Radiation detection§ Neutron activation for 227Ac
à 1E-17 g level§ Activity used to determine to 1E-20 g
3-15
Actinium uses
• Heat sources§ 227Ac multiCi amount§ 5 alpha particles
• Neutron sources§ a,n source using 227Ac
• Nuclear medicine§ Medical use based on ligand
à Bone treatments (polyphosphonate )§ 225Ac suitable isotope
à Decay series produces alpha and beta* No hard gammas
§ Ac bound by marcocyclic compounds§ HEHA incorporates Ac
à 1,4,7,10,13,16-hexaazacyclohexadecane-N,N′,N′,N′,N′,N′-hexaacetic acid
• Geotracer§ Compare 231Pa with 227Ac
à Ac higher than Pa in deep seawaterà Ac as a tracer for deep seawater circulation
3-16
Review• Actinide isotopes
§ Lifetimes, production• Actinium data
§ X-ray, radii, density• Redox
§ Oxidation states• Preparation and purification
§ Solvent extraction and ion exchange• Compounds and properties• Uses of actinium
3-17
Questions
• What is the longest lived Ac isotope?• What are two different ways to obtain actinium
§ Provide the isotopes• What Ac oxidation state can be found in
solution?• How is Ac separated from U• Which Ac species are insoluble?• What are some uses of Ac?
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